effect of painting on the mechanical properties of injection-molded plastics
DESCRIPTION
Paint can have significant effects on the mechanical properties of plastic substrates. The selection of a paint system that is not properly matched to a given plastic substrate can lead to premature failure. While anecdotal accounts are plentiful, quantitative data regarding effects of coating on plastics is relatively scarce. This paper describes the effect of a high-solids acrylic topcoat paint, with either of two different adhesion promoters or no adhesion promoter, on the mechanical properties of four different plastic substrates: general-purpose acrylonitrile-butadiene-styrene (ABS), high-impact ABS, weather-resistant acrylonitrile-styrene-acrylate (ASA), and mineral-filled polyethylene ionomer/polyamide-6 blend.TRANSCRIPT
Effects of Painting on Mechanical Properties of Injection Molded Plastics
David E. Palmer, P.E.BRP – Marine Propulsion Systems Division,
Sturtevant, WI
Materials and coatingsQUESTION: Why apply a coating to a material?
2
Appearance
Abrasion/wear
Friction coefficient
Thermal barrier
Electrical conductivity
Corrosion resistance
Chemical resistance
UV resistance
Static dissipation
Optical reflectivity
Materials and coatings
3
QUESTION: Why apply a coating to a material?
ANSWER:
To change the material’s properties and/or behavior
Materials and coatings
4
… is it reasonable to assume that all other aspects will
remain unchanged?
QUESTION: If we change one aspect of a materials’ properties or behavior…
Materials and coatings
5
Effect of nickel plating on fatigue life of steel
Source:J.O. Almen, "Fatigue, loss
and gain by electroplating," Product Engineering 22:109-116,
June 1951.
Materials and coatings
6
Effect of zinc-plated nut on glass-filled nylon-6,6
EDS of corrosion product on surface of nut
Paint and plastics
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QUESTION: Why paint plastics?
Mass customization
Color/effect not available in resin
Resin lacks UV resistance
Aftermarket refinishing
Touchup/repair
Paint and plastics
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QUESTION: What are the effects of painting on the mechanical properties of plastics?
•Mechanical interactions
•Chemical interactions
•Thermal effects
Mechanical interactions• Elastic mismatch• Interfacial strength• Film thickness
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Source:M.S. Hu and A.G. Evans,
"The cracking and decohesion of thin films on ductile substrates,"
Acta Metallurgica 37:917-925, 1989.
Chemical interactions
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• Swelling• Dissolution• Environmental stress cracking
Thermal effects
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• Accelerated solvent diffusion into substrate• Relief of molded-in stresses• Shrinkage of paint (induces residual stresses)• Thermal degradation of substrate
Experimental design• For four (4) different substrates, measure
mechanical properties in each of these six (6) conditions:
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No coating Adhesion promoter 1 only
Adhesion promoter 2 only
Topcoat onlyAdhesion
promoter 1 with topcoat
Adhesion promoter 2 with
topcoat
Substrates
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• General purpose ABS
• High impact ABS
• Weather-resistant ASA
• 10% mineral filled polyethylene ionomer/PA-6 blend
Coatings
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• High-solids black acrylic topcoat
• Clear adhesion promoter (40% aromatic hydrocarbons)
• Black adhesion promoter (<2% aromatic hydrocarbons)
Sample preparation• Black topcoat:
45 µm dry film build
Cure at 93°C for 20 minutes
• Clear adhesion promoter:5 µm dry film build
Cure at 77°C for 20 minutes
• Black adhesion promoter:20 µm dry film build
Cure at 71°C for 20 minutes
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Testing
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• Notched Izod impactASTM D256-10 Method A
• Tension testingASTM D638-10
Polyethylene ionomer/PA-6 blend: 50 mm/min
All other substrates: 5 mm/min
Notched Izod impact
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Tensile modulus
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Tensile strength
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Strain at yield
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Strain at break
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Supression of necking
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High impact ABSBlack adhesion promoter and
topcoat
High impact ABSUncoated
This effect was observed in all of the coated ABS and ASA specimens.
Film cracking
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This effect was observed in all of the ABS and ASA
specimens with topcoat (regardless of whether or not they had adhesion
promoter).
Loss of adhesion
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This effect was observed the PE ionomer/PA-6
specimens with topcoat (regardless of whether or
not they had adhesion promoter).
Film adhesion
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For the PE ionomer/PA-6 specimens with either adhesion promoter (without topcoat), no loss of
adhesion was observed.
The specimen above experienced 165% elongation at break.
Conclusions
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Rank ordering of effects:
• Strain at break (most affected by painting)• Impact strength• Tensile modulus• Tensile strength• Strain at yield (least affected by painting)
Conclusions
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Coating effect on substrates:
For ABS and ASA substrates, strain at break and impact strength decreased with coating.
For mineral-filled PE ionomer/PA-6 blend, strain at break and impact strength increased with coating.
Conclusions
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Effects of adhesion promoters/topcoat:
The effects of the topcoat and the two adhesion promoters were different on different substrates.
When the adhesion promoters were used together with the topcoat, the effects were not additive.
Conclusion
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Matching a paint system to a plastic substrate should be done with great care.
Published data is scarce.
Testing is the best way to ensure a successful match.
Acknowledgements
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PolyOne – Richard Baril
SABIC – Dennis Yon
LTL Color Compounders – Don Hone
PPG Industries – Ron Kalski, Dave Nale, Cari Stein
Polymer Diagnostics – Keith Mathews, Jill Moran, Katlyn Hoffman
BRP US, Inc. – Bill Barth, Rich Smock, Mike Davenport, Ben Jones, Matt Coyne